1. Field of the Invention
The present invention relates generally to a stack of fuel cells which are capable of converting the latent chemical energy of a fuel into electricity and, more particularly, to a manifold and manifold seal used in conjunction with such a stack of fuel cells.
2. Description of the Prior Art
Fuel cells used to convert the latent chemical energy of a fuel directly into electricity are well-known in the art. For example, see U.S. Pat. No. 4,463,068. Such cells may be based on a variety of electrochemical reactions. One well-known reaction is based on using hydrogen as a fuel which reacts with oxygen to generate electricity.
One common form for constructing a hydrogenoxygen cell is a laminated structure wherein an anode electrode and a cathode electrode are spaced apart by a porous layer of material which holds an electrolyte such as concentrated phosphoric acid. The hydrogen is guided by passageways behind the active region of the anode and the oxygen is guided by passageways behind the active region of the cathode. Both the anode and the cathode have a catalyst, such as platinum, deposited thereon.
At the anode, the hydrogen gas dissociates into hydrogen ions plus electrons in the presence of the catalyst. The hydrogen ions migrate through the electrolyte to the cathode in a process constituting ionic current transport while the electrons travel through an external circuit to the cathode. At the cathode, the hydrogen ions, electrons and molecules of oxygen combine to produce water.
It is known to orient the passageways behind the anode and cathode so as to have all of the inputs and outputs for both the oxidant and fuel oriented such that all of the fuel cells may be serviced by common manifolds. The sealing of such a manifold to a stack of fuel cells presents a variety of problems. First, the sealing surface of the stack of fuel cells presents an irregular, stepped surface due to manufacturing and stacking tolerances of the various components of the fuel cells. In addition to conforming to the irregular stack surface, the seal must withstand pressures of up to approximately five psi. Over the operational life of the fuel cell stack, the fuel cell stack tends to shrink, or creep. For a current 32 kilowatt 152 cell stack design, that creep is about one and one-half inches (3.8 cms). For a 100 kilowatt stack, the creep is about four and one-half inches (11.43 cms.) The manifold seal must remain operative as the fuel cell stack creeps over its operating life.
The manifold and seal must be capable of operating at temperatures of approximately 375.degree. F. Where the electrolyte used by the fuel cell is phosphoric acid, the seal must be resistant to hot phosphoric acid and steam. The seal leakage should be less than 0.2% of flow at a differential pressure of approximately one-half psi. The manifold and seal assembly must accommodate stack bowing and maintain near constant seal loading.
The prior art has attempted to address those problems. In U.S. Pat. No. 4,212,929, a Fuel Cell Manifold Sealing System is disclosed. The sealing system includes a polymer seal frame firmly clamped between the manifold and the stack such that the seal frame moves with the stack. Thus, as the stack creeps, the seal frame creeps with it. Therefore, there is no sliding at the rough stack to seal frame interface. All of the sliding takes place on the smooth location between the seal frame and the manifold. Such a sealing system, however, creates two areas which must be sealed. The first is the area between the seal frame and the stack, which is a fixed, rigid connection, and the second is the area between the seal frame and the manifold which is a sliding type of connection. It is, therefore, desirable to have a manifold and seal which are capable of operating in the harsh environment presented by a stack of fuel cells over the life of the stack in a simple, yet reliable, manner.